It is known in the art that vaporized fuel generated in a fuel tank is temporally absorbed in a canister and a purge control valve is opened depending on an operational condition of an engine, so that the vaporized fuel absorbed in the canister is supplied into an intake system of the engine together with fresh air because of negative pressure generated by the engine. As a result, the vaporized fuel is processed by combustion in the engine
As disclosed in Japanese Patent Publication No. 2003-74423 (hereinafter, a prior art document No. 1), in the above vaporized-fuel processing system, an abnormal diagnosis is carried out for the purge control valve whether the purge control valve is normally operated or not. According to the above prior art document No. 1, a control amount for an air-fuel ratio is calculated during a process for increasing an amount for a purging operation and the purge control valve is determined as being fixed to its valve closed position, when such calculated control amount for the air-fuel ratio is lower than a predetermined value.
Recently, an engine having a supercharging device comes under a spotlight in view of a down-sizing of an engine. In case of such engine, it is not possible to supply vaporized fuel into an intake system of the engine by use of negative pressure generated by the engine. It is, therefore, anticipated that an opportunity for processing the vaporized fuel is reduced.
Such a vaporized-fuel processing system is developed, for example, as disclosed in the following prior art document No. 2:
“2011 MY OBD System Operation Summary for Gasoline Engines. [Online].Ford Motorcraft.Com. Retrieved from the Internet: <URL: http://www.motorcraftservice.com/vdirs/retail/default.asp?pagei d=diag_theory_retail&gutsid=diagsheet&kevin=rules>”
According to the above vaporized-fuel processing system, there are provided with two purging systems. According to one of the purging systems, the vaporized fuel is supplied into the intake system of the engine by use of the negative pressure via a first purge passage, during an engine operation in which a supercharging operation is not carried out for the intake air. According to the other one of the purging systems, the vaporized fuel is supplied into the intake system of the engine via a second purge passage, during an engine operation in which the intake air is supercharged.
More in detail, according to the above prior art document No. 2, the purge passage for connecting a canister and the intake system of the engine has the first and the second purge passages, wherein the first purge passage is connected to an intake-air passage at a downstream side of a throttle valve, while the second purge passage is connected to the intake-air passage at an upstream side of an intake-air compressor of the supercharging device. The purge control valve is provided in the purge passage, which is commonly used for the first and second purge passages. In addition, a first check valve, which is opened by the negative pressure in the intake-air passage, is provided in the first purge passage. An ejector, which is operated by supercharged intake air, and a second check valve, which is opened by operation of the ejector, are provided in the second purge passage.
According to such a system, the vaporized fuel absorbed in the canister is purged into the intake-air passage by the negative pressure of the engine, via the first purge passage, when the purge control valve is opened during the engine operation in which the supercharging operation is not carried out. On the other hand, the vaporized fuel absorbed in the canister is further purged into the intake-air passage of the engine by the operation of the ejector, via the second purge passage, when the purge control valve is opened during the engine operation in which the supercharging operation is carried out.
The above prior art document No. 2 further discloses that abnormal diagnosis is carried out for the check valves provided in the first and the second purge passages (more exactly, whether the first check valve in the first purge passage is fixed to its valve opened condition, and whether the second check valve in the second purge passage is fixed to its valve closed position), based on a changing ratio of a fuel pressure in the fuel tank in the condition that an air-communication valve for the canister is closed while the purge control valve is opened.
According to the above prior art document No. 2, however, it is only possible to diagnose specific abnormal conditions of the check valves provided in the purge passages. In other words, it is not possible to diagnose such abnormal conditions whether the first check valve provided in the first purge passage is fixed to its valve closed position or whether the second check valve provided in the second purge passage is fixed to its valve opened position. It is regulated by law for the vaporized-fuel processing system having two purge passages to carryout flow-check for the respective valves provided in each purge passage. Namely, it is necessary to identify which of the valves is not normally operated and whether such valve is fixed to its valve closed position or to its valve opened position.